By contrast, solid-state NMR spectra are very broad, as the full effects of
anisotropic ... Much of the original solid state NMR in the literature focuses only
upon the ...
Introduction to Solid State NMR In solution NMR, spectra consist of a series of very sharp transitions, due to averaging of anisotropic NMR interactions by rapid random tumbling. By contrast, solid-state NMR spectra are very broad, as the full effects of anisotropic or orientation-dependent interactions are observed in the spectrum. High-resolution NMR spectra can provide the same type of information that is available from corresponding solution NMR spectra, but a number of special techniques/equipment are needed, including magic-angle spinning, cross polarization, special 2D experiments, enhanced probe electronics, etc. The presence of broad NMR lineshapes, once thought to be a hindrance, actually provides much information on chemistry, structure and dynamics in the solid state.
Solution 13C NMR
Solid State 13C NMR
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Origins of Solid-State NMR Original NMR experiments focused on 1H and 19F NMR, for reasons of sensitivity. However, anisotropies in the local fields of the protons broadened the 1H NMR spectra such that no spectral lines could be resolved. The only cases where useful spectra could be obtained was for isolated homonuclear spin pairs (e.g., in H2O), or for fast moving methyl groups. Much of the original solid state NMR in the literature focuses only upon the measurement of 1H spin-lattice relaxation times as a function of temperature in order to investigate methyl group rotations or motion in solid polymer chains. The situation changed when it was shown by E.R. Andrew and I.J. Lowe that anisotropic dipolar interactions could be supressed by introducing artificial motions on the solid - this technique involved rotating the sample about an axis oriented at 54.74° with respect to the external magnetic field. This became known as magic-angle spinning (MAS). static (stationary sample) 19
F NMR of KAsF6 1 75 J( As,19F) = 905 Hz Rdd(75As,19F) = 2228 Hz MAS, 1/2, and an asymmetric distribution of nucleons giving rise to a non-spherical positive electric charge distribution; this is in contrast to spin-1/2 nuclei, which have a spherical distribution of positive electric charge.
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nuclear charge distribution
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+
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+
+
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_
+ Spin-1/2 Nucleus
electric field gradients in molecule
Quadrupolar Nucleus
The asymmetric charge distribution in the nucleus is described by the nuclear electric quadrupole moment, eQ, which is measured in barn (which is ca. 10-28 m2). eQ is an instrinsic property of the nucleus, and is the same regardless of the environment. prolate nucleus
oblate nucleus
eQ > 0
eQ < 0
Quadrupolar nuclei interact with electric field gradients (EFGs) in the molecule: EFGs are spatial changes in electric field in the molecule. Like the dipolar interaction, the quadrupolar interaction is a ground state interaction, but is dependent upon the distribution of electric point charges in the molecule and resulting EFGs.
Solid-State NMR of Quadrupolar Nuclei The EFGs at the quadrupolar nucleus can be described by a symmetric traceless tensor, which can also be diagonalized: Vxx Vxy Vxz V ' Vyx Vyy Vyz
Vzx Vzy Vzz
VPAS '
V11
0
0
0
V22
0
0
0
V33
The principal components of the EFG tensor are defined such that *V11* # *V22* # *V33*. Since the EFG tensor is traceless, isotropic tumbling in solution averages it to zero (unlike J and F). The magnitude of the quadrupolar interaction is given by the nuclear quadrupole coupling constant: CQ = eQ@V33/h (in kHz or MHz) The asymmetry of the quadrupolar interaction is given by the asymmetry parameter, 0 = (V11 - V22)/V33, where 0 # 0 # 1. If 0 = 0, the EFG tensor is axially symmetric. For a quadrupolar nucleus in the centre of a spherically symmetric molecule, the EFGs cancel one another resulting in very small EFGs at the quadrupolar nucleus. As the spherical symmetry breaks down, the EFGs at the quadrupolar nucleus grow in magnitude: NH3 Cl Cl NH3 NH3 NH3 NH3 NH3 NH3 Co Co Co Br NH3 NH3 NH3 NH3 NH3 NH3 Br NH3 Increasing EFGs, increasing quadrupolar interaction
Solid-State NMR of Quadrupolar Nuclei The quadrupolar interaction, unlike all of the other anisotropic NMR interactions, can be written as a sum of first and second order interactions: (1)
(2)
,Q ' , Q % , Q
Below, the effects of the first- and second-order interactions on the energy levels of a spin-5/2 nucleus are shown: mS -5/2 -3/2 -1/2 +1/2 +3/2 +5/2 ,Z
(1)
,Q
(2)
,Q
The first order interaction is proportional to CQ, and the secondorder interaction is proportional to CQ2/
